A clause is a group of words that includes a subject and a verb.
A clause forms a sentence or part of a sentence.
A main clause is a clause which has meaning without the help of another clause.
A main clause can stand alone as a sentence.
A sentence may contain two or more clauses.
A sentence may have one main clause and more than one subordinate clauses.
A main clause is often combined with a subordinate clause.
A subordinate is a group of words that is not a sentence but adds information to the main part of a sentence.
Therefore, a subordinate clause cannot stand alone as a sentence.
A main clause may be sandwiched between two subordinate clauses.